The present invention relates to a heat-sensitive recording material which makes a record of various types of information by thermal means.
In certain types of information equipments such as facsimilies, printers, recorders or the like, a system that a record is made of information by thermal means has been developed in recent years. Keeping in step with the advent of such a system, it has been proposed to make use of a variety of heat-sensitive recording materials. Among others, noteworthy is now a heat-sensitive recording material comprising a carrier or substrate having thereon a heat-sensitive layer composed mainly of a leuco dye that is colorless in a normal state and a developer such as a phenolic compound.
The article "Berichte der deutschen chemisehen Gesellschaft" by O. Fisher, F. Romer et al., 42.2934 (1909) already describes that a reaction between a chromogenic leuco dye, e.g., crystal violet lactone and a phenolic compound results in color development. The color development caused by thermal means is also known from, for example, U.S. Pat. No. 3,539,375 and Japanese Patent Publication No. 14039/1970 specifications.
In the above-mentioned type of heat-sensitive recording materials, a binding agent generally takes an important role in addition to a color-forming component. Usually, the heat-sensitive recording materials are prepared by dispersing individually a leuco dye serving as a color former and a developer in separate aqueous medium containing a water-soluble resin acting as a binding agent by a variety of dispersing means such as ball mills, sand grinders or the like for fine pulverization thereof, mixing together the resulting dispersions, and applying the mixture on a carrier followed by drying. The binding agent used to this end should possess the properties as discussed below:
(1) it is readily dispersed; in other words, it suffers neither interaction with the color former nor aggregation, and does not produce a large amount of foams;
(2) upon dispersion, any coloring, aggregation and viscosity rises of the liquid are not observed in admixing of both components; and after coating and drying,
(3) the resulting film has a great strength;
(4) no primary color development takes place;
(5) it excels in the color-forming properties, possesses a good thermal conductivity and is free from any desensitization; and
(6) it is free from any sticking and deposition, and displays a good matching with respect to a thermal head.
As the binding agent, use has heretofore been made of water-soluble polymeric compounds such as polyvinyl alcohol, (modified) starch or its derivatives, methyl cellulose, hydroxylethyl cellulose, carboxylmethyl cellulose, gum arabic, gelatin, casein, polyvinyl pyrolidone, polyacryl amide, polyacrylate, copolymers of styrene and maleic anhydride, copolymers of isobutylene and maleic anhydride etc., and polymeric latexes or emulsions such as copolymers of styrene and butadiene, polyvinyl acetate, polyacrylic ester etc. However, no binding agent which meets all the foregoing requirements as such has been found as yet. For example, the methyl cellulose, hydroxyethyl cellulose and carboxylmethyl cellulose are disadvantageous in that they are poor in the application and dispersing properties or the color-forming properties upon coating. This is because they have a high viscosity at a low conentration, so that the resulting dispersion is of a high viscosity. The polyvinyl pyrolidone and most of other thermoplastic resins have a softening point of no more than 200.degree. C. and inferior in the heat resistance, so that undesirable sticking is apt to take place at a thermal head.
The latexes or emulsions are also unpreferable since they are unsatisfactory in the dispersing properties upon dispersion, so that the primary color development is readily caused under the influence of certain emulsifiers contained therein. Moreover, the gum arabic, gelatin, casein or the like have various diadvantages such as occurrence of the primary color development and sticking.
Although the polyvinyl alcohols or modified starch such as oxidized or etherized starch appear to be most preferable binding agents in view of the aforesaid requirements, there is still room for improvements in respect of the primary color development.
As a consequence of extensive investigations and studies, it has now been found that a binding agent comprising two through four components selected from the group consisting of any one or combinations of polyvinyl alcohol, oxidized starch and etherized starch, and methyl cellulose in a ratio by weight between 98:2 and 90:10 meets all the aforesaid requirements, and is thus of extremely high quality. More specifically, it has been found that the binding agent comprising a mixture of any one or combinations of polyvinyl alcohol, oxidized starch and etherized starch with methyl cellulose in the above-mentioned weight ratio does not only comply with the requirements 1, 2, 3 and 6, but also provides a heat-sensitive recording material which suffers no primary color development, displays a brightness close to that of natural paper and is excellent in the color development.